A certain apparatus for machining a workpiece to yield a product with curved surfaces that are non-circular in cross section, as exemplified by a cam, piston or piston ring having a freely curved or sculptured surface, has so far been known. For example, JP S54-21691 A, JP S63-22201 A and JP H6-75814 B describe such apparatus that yield such products with precision as desired.
The machining apparatus disclosed in JP S54-21691 A is designed to produce a stack of piston rings and is described as characterized by including a workpiece support shaft adapted to carry a workpiece constituted by a piston ring workpiece aggregate comprising a plurality of piston ring workpieces stacked and held together, a tool support means for supporting a cutting tool so as to be radially displaceable in accordance with circularities of machinable surfaces of the piston rings, and at least two electro-mechanical stepping feed means that are with varying amounts of stepping feed and are computer controllable to displace the tool support means radially as desired. In the apparatus so constructed, rotating the workpiece support shaft with the piston ring aggregate carried thereon while driving the stepping feed means under computer control is described to provide machining of the workpieces with their inner and outer peripheral surfaces defined by free curves machined simultaneously with precision as sought.
JP S63-22201 A discloses a lathe with cutting tools for use to machine inner and outer surfaces of a piston ring workpiece, the lathe being designed so that displacing these tools with rotating cams radially of the piston ring workpiece may have the inner and outer peripheral surfaces formed simultaneously of a piston ring product.
The numerically controlled lathe disclosed in JP H675814 B includes a cutter mounting carriage that is movable by a linear motor towards and away from a workpiece such as to yield a piston ring with a skirt portion, the linear motor being adapted to be driven under numerical control by a computer to have an outer peripheral surface of the skirt portion formed by lathing the workpiece. The lathe has a guide section for supporting the carriage that reciprocates, and a plurality of sets of juxtaposed rotary members operable at the opposite sides of the carriage. Also included is a biasing means that supports one of the sets of rotary members to prevent the carriage from laterally moving so that a reaction force that when the workpiece being lathed exerts on the cutting tool may be accepted by both the rotary members and the biasing means. This provision is described to permit the carriage to reciprocate without jolting or shaking, which in turn provides, among others, permitting workpieces to be cut with precision.
The machining apparatus described in JP S54-21691 A, however, in which a piston ring workpiece aggregate is mounted on the workpiece support shaft that extends in a horizontal direction and is rotated therewith to have their outer and inner surfaces simultaneously cut, it has been found that chips caused in cutting its inner surface areas tend to stay inside of the workpiece. This inconveniently requires means for ejecting the chips to be additionally provided and, if they fail to be ejected while cutting is allowed to continue, permits the chips, physically to lower, or otherwise, by raising the temperature of the workpiece with their developing heat, to adversely affect, the cutting accuracy of the workpiece machined.
Also, a plurality of piston ring workpieces in order to be retained integrally in their aggregate form need to be compressed under an elevated pressure typically amounting to as high as 10 tons from its opposite sides. Then, if the workpiece should be supported to extend horizontally as taught in JP S54-21691 A, a very rigid mechanical structure is required to accommodate such a force of compression that is applied horizontally. Inconveniently, this renders the equipment large sized and costly.
Further, if the workpiece machined is an aggregate or stack of elements such as piston rings each with a given phase orientation which once shaped must be cut and divided or split in a subsequent machining step, it must be carried out with these shaped elements held to retain their oriented phase relationship.
To that end it has been customary to carry out such a shaped workpiece with its opposite ends held clamped with a hand carrier jig. However, the workpiece if constituted by piston rings stacked in a horizontal direction as taught in JP S54-21691 A is found to make the hand carrier jig poorly detachable. Also, the gravity that acts orthogonally to the direction in which the piston rings constituting the shaped workpiece are stacked is found to inconveniently cause, as it is carried out, the piston rings to easily get phase shift.
The machining apparatus described in JP S54-21691 A,especially in connection with FIG. 5, further requires that the tool support means having a cutting tool for machining the outside of a workpiece be mounted on a sliding head of which a movement is controlled by a pair of step feed means that are arranged back and forth lengthwise of it. The sliding head has on it a second sliding head that is capable of movement by the step feed means in the same direction as that in which the first sliding head is movable. And, a tool support means having a cutting tool for machining the inside of the workpiece is required to be mounted on the second sliding head. The apparatus so constructed of inconveniently has, of necessity, drive systems interlaced and complicated and, also complicating control units therefor, renders the entire equipment costly.
The lathe described in JP S63-22201 A employs a cam arrangement that mechanically effects feeding cutting tools to shape or machine the inside and outside of a workpiece, respectively. The lathe thus inconveniently requires the cams to be replaced each instance a piston ring geometry is altered. The replacement of such cams is time consuming and of poor efficiency. The need to prepare a number of cams that vary in geometry makes their maintenance and management laborious and inefficient, and raises the cost of production of piston rings.
Such a structure is also inconvenient in that a cam wears in servicing and a drive system in operation tends to develop a jolting, which makes it difficult to machine a workpiece with accuracy. A machining inaccuracy so caused by such as cam wear inconveniently cannot simply be compensated for.
The NC lathe described in JP H6-75814 B is not adapted to machine both inside and outside of a workpiece simultaneously and thus is unsatisfactory in productivity. The requirement for the inside and outside of a workpiece to be separately shaped adversely affects their machined concentricity and here again leaves much to be desired in achievable machining accuracy.
It is accordingly an object of the present invention to eliminate these disadvantages met in the prior art and to provide an apparatus that is capable of machining both inside and outside of a hollow workpiece simultaneously and quickly to impart thereto inner and outer curved surfaces that are non-circular in cross section with an increased precision, the apparatus having its production reduced and being rendered over its entire equipment less costly.